CELL-CYCLE, DIFFERENTIATION AND TISSUE-INDEPENDENT EXPRESSION OF RIBOSOMAL-PROTEIN L37

A unique human cDNA (hG1.16) that encodes a mRNA of 450 nucleotides wa s isolated from a subtractive library derived from HeLa cells. The rel ative expression level of hG1.16 during different cell-cycle phases wa s determined by Northern-blot analysis of cells synchronized by double -thymidine block and serum deprivation/refeeding. hG1.16 was constitut ively expressed during all phases of the cell cycle, including the qui escent phase when even most constitutively expressed genes experience some suppression of expression. The expression level of hG1.16 did not change during terminal differentiation of myoblasts to myotubes, duri ng which cells become permanently post-mitotic. Examination of other t issues revealed that the relative expression level of hG1.16 was const itutive in all embryonic mouse tissues examined,including brain, eye, heart, kidney, liver, lung and skeletal muscle. This was unusual in th at expression was not down-modulated during differentiation and did no t vary appreciably between tissue types. Analysis by inter-species Nor thern-blot analysis revealed that hG1.16 was highly conserved among al l vertebrates studied (from fish to humans but not in insects). DNA se quence analysis of hG1.16 revealed a high level of similarity to rat r ibosomal protein L37, identifying hG1.16 as a new member of this multi gene family. The deduced amino acid sequence of hG1.16 was identical t o rat ribosomal protein L37 that contained 97 amino acids, many of whi ch are highly positively charged (15 arginine and 14 lysine residues w ith a predicted M(r) of 11065). hG1.16 protein has a single C-2-C-2 zi nc-finger-like motif which is also present in rat ribosomal protein L3 7. Using primers designed from the sequence of hG1.16, unique bovine a nd rat cDNAs were also isolated by 5'-rapid-amplification of cDNA ends . DNA sequences of bovine and rat G1.16, clones were 92.8% and 92.2% s imilar to human G1.16 while the deduced amino acid sequences derived f rom bovine and rat cDNAs each differed by a single amino acid from the sequence of hG1.16 and the published rat L37 sequence. Southern-blot analysis revealed that hG1.26 exists in multiple copies in human. rat and mouse genomes. These G1.16 clones encode unique human, rat and bov ine members of the ribosomal protein L37 gene family, which are consti tutively expressed even during transitions from quiescence to active c ell proliferation or terminal differentiation, in all tissues and all vertebrates investigated.